Magnetic electron lens



y 1957 A. RADEMAKERS ET AL 2,799,813

MAGNETIC ELECTRON LENS Filed May '7, 1955 INVENTORS ADRIAAN RADEMAKERS EDUARD JQHAN HAES United States PatentCfiice Patented July 16, 1957 MAGNETIC ELECTRON LENS Adriaan Rademakers and Eduard Johan Haes, Em-

masingel, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application March 7, 1955, Serial No. 492,542

Claims priority, application Netherlands March 5, 1954 Claims. (Cl. 317200) The invention relates to a magnetic electron lens comprising two opposite circular rings of permanent magnetic material. A known embodiment of such electron lenses has the rings arranged such that their direction of magnetisation extends radially or else axially relative to the axis of the rings. The first-mentioned method of magnetisation has the disadvantage that the stray field on the axis beyond the magnets is larger than with the last-mentioned method of magnetisation, though it has the advantage that a smaller quantity of magnetic material is re quired to obtain the same lens intensity.

The invention has for its object to provide an electron lens in which the magnet rings are magnetized in a manner such that even a smaller quantity of magnetic material is required. It is characterized in that the inner cylindrical surface of one ring constitutes a pole surface having south magnetism and that of the other ring constitutes a pole surface having north magnetism, whereas the two surfaces of the rings facing one another constitute the pole surfaces of opposite magnetism.

The invention will now be described with reference to the accompanying drawing.

Fig. 1 shows a magnetic electron lens according to the invention;

Fig. 2 shows graphs to explain Fig. 1;

Fig. 3 shows a variant of the embodiment shown in Fig. 1;

Fig. 4 shows a polarisation device for the magnets shown in Fig. 1;

Fig. 5 shows a magnetic electron lens comprising means to compensate the temperature dependence of the lens intensity;

Fig. 6 shows, in cross-section, a modification of the device illustrated in Fig. 5.

Fig. 1 shows a magnetic electron lens comprising two opposite circular rings 1 and 2 of permanent magnetic material, for example a ceramic ferromagnetic material composed of iron oxide and an oxide of barium, strontium or lead and having a hexagonal crystal structure, which may for example be applied around the neck of a cathode-ray tube. In contradistinction to the known embodiments, in which the rings were magnetized radially or else axially, the rings in this case have a magnetisation I, wherein the inner cylindrical surface 3 of the ring 1 constitutes a pole surface having south magnetism, the inner cylindrical surface 4 of the ring 2, on the contrary, constitutes a pole surface having north magnetism, whereas the two opposite surfaces 5 and 6 of the rings 1 and 2 respectively constitute pole surfaces of opposite magnetism, i. e., northand south-magnetism respectively.

In Fig. 2 the curves illustrate the magnitude of the magnetic field strength H, measured along the optical axis 7 of the lens illustrated in Fig. l with two different values (0 and 8 millimeters) of the distance 1 between the two rings 1 and 2, the curves r relating to the case in which the rings are magnetized radially in accordance with the known embodiment, whereas the curves s relate to the case in which they are magnetized in accordance with the invention.

It is evident therefrom that the field strength H and hence the lens intensity, which is proportional to H dz, wherein z designates the coordinate along the optical axis 7, remains smaller in the case of radial magnetisation than in the case of magnetisation in accordance with the invention. This means that in the latter case the generation of a given lens intensity requires a smaller quantity of magnetic material than in the former case. It is true that the stray field on the axis '7 beyond the magnets 1 and 2 is in this case slightly larger, but this field may, if necessary, be restricted with the aid of a disc 8 of high permeable material, for example soft iron or ferrite, arranged, if necessary, inside the cathode-ray tube.

The said economy in material is found to be of particular importance, if the axial thickness d differs little from the radial thickness h of the rings; their ratio lies preferably between the values 2/3 and 3/2, in which case at least 20% of the permanent magnet material may be economized. Moreover, it is found that the material in the proximity of the corners or edges 9 and 1t), remote from the pole surfaces 3, 5 and 5, 6 respectively, contributes to a smaller extent to the field H on the axis 7 than the further material. Fig. 3 shows therefore an embodiment, wherein these corners are bevelled, which provides a further economy of material of another 20%.

Fig. 4 shows a polarisation device for producing the magnetisation I of Fig. 1. It comprises two co-axial magnet coils 13 and 14, between which the magnet 1 to be polarized is arranged and which, for example by inverting the sense of winding and/or by a different choice of the number of turns, produce rotational-symmetrical fields of different values in the space between the two coils, of which the resultant Hp lies obliquely to the axis 7 of the coils, and which resultant produces, consequently, the desired magnetisation I in the magnet 1.

The lens intensity of the electron lenses shown in Figs. 1 and 3 is found to drop, in general, with temperature, since the field produced by the magnet rings 1 and 2 drops, as a rule, with increasing temperature. This drop in lens intensity may be compensated, according to Fig. 5, by means of a spring 17, which keeps the rings 1 and 2 spaced apart and of which the rigidity has a prescribed value, which is chosen such that the magnets 1 and 2 are spaced apart by a desired, predetermined distance, dependent upon the lens intensity desired along the axis. In other words, the spring 17 is adjusted to provide an expansion force tending to separate the magnets 1 and 2, to offset the magnetic attractive forces tending to bring the magnets 1 and 2 together. When the temperature increases, there results a decrease in the magnetic field produced by the rings 1 and 2, which affects not only the lens intensity but also the force with which the rings 1 and 2 attract one another. However, due to the presence of the spring 17, the distance between the rings is increased, which implies an increase in lens intensity, which tends to compensate the said decrease in the case of suitable proportioning of the elements.

A modification of this structure is shown in Fig. 6, wherein a spring 22 is arranged, preferably, as a push spring or a tensile spring, between the magnet ring 1 and a member 21 at some distance from the fixed magnet ring 2', this member 21 being stationary. The magnet 1 is mounted on a support 20, which is axially displaceable. Thus, the position of the magnet 1', which determines the lens intensity, depends on the force imparted by the spring 22 and the strength of the attractive magnetic force be tween the magnets 1' and 2. The said compensation method may even be carried out, if the rings 1 and 2 are magnetized in a radial direction, i. e. in general, if the rings 1 and 2 attract one another.

What is claimed is:

1. A magnetic electron lens comprising a pair of substantially fiat, coaxial, adjacent, annular, permanent magnet members, each of said annular members having a given thickness in an axial direction and a given thickness in a radial direction between its inner and outersurfaces, one of said members being magnetized obliquely forming a south pole on its inner surface and a north pole on the substantially flat surface thereof facing said other member, the other member being magnetized obliquely forming a north pole on its inner surface and a south pole on the substantially fiat surface thereof facing said one member, said pair of members cooperating to produce a magnetic field substantially along the axis thereof.

2. A magnetic electron lens as claimed in claim 1 wherein the ratio between the given thickness in the axial direction and the given thickness in the radial direction of each of the members is in the range between 2/3 and 3/2.

3. A magnetic electron lens as claimed in claim 1 wherein the edges of the annular members remote from their facing surfaces and their axis are bevelled.

4. A magnetic electron lens comprising a pair of substantially flat, coaxial, spaced, annular, permanent magnet members, said magnet members being magnetized to produce opposite polarity poles on facing surfaces thereof thereby producing attractive forces therebetween, said members producing magnetic fields whose magnitude is temperature dependent, and resilient means for maintaining said members spaced apart, said resilient means producing an expansion force at least partly counteracting the attractive forces between the magnets.

5. A magnetic electron lens as set forth in claim 4 wherein the resilient means is a tensile spring.

References Cited in the file of this patent UNIT ED STATES PATENTS 2,l57,l82 Malofi May 9, 1939 2,339,087 Mantz Jan. 11, 1944 2,525,919 Loughren Oct. 17, 1950 

